The exhaust gas emitted from an internal combustion (IC) engine, is a heterogeneous mixture that may contain gaseous emissions such as carbon monoxide (CO), unburned hydrocarbons (HC) and oxides of nitrogen (NOx) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Typically, catalyst compositions disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Exhaust gas treatment systems typically include selective catalytic reductant (SCR) devices to reduce the amount of NOx in the exhaust gas. The typical exhaust treatment system also includes a reductant injection system that injects a reductant such as, for example, ammonia (NH3). The SCR devices make use of NH3 to reduce the NOx. For example, when the proper amount of NH3 is supplied to the SCR device under the proper conditions, the NH3 reacts with the NOx in the presence of an SCR device to reduce the NOx emissions. However, if the reduction reaction rate is too slow, or if there is excess NH3 in the exhaust, NH3 can slip from the SCR device and be emitted to the environment.